"We want strategic diversity for season 5"
Continues to ignore the fact that marksmen are the only 100% mandatory role
Inb4 "but supports are also mandatory" Annie is not a support, fiddlesticks is not a support, yet they've both shown in the past that they've been the meta bot lane. Just because someone is being played with 0 farm doesn't change their champion class. Hypothetically speaking if Poppy went bot lane with a marksmen she'd still be a fighter not a support lol
Inb4 "but junglers are also mandatory" Jungle is a position on the map, not a class of champs lol. This is the dumbest argument I've ever heard. Top laner is not a class, it's a role that consists of fighters, mages, tanks, etc. Jungler is not a class of champions, it's a role that consists of tanks, fighters, etc. Mid laner is not a class, it's a role that consists of assassins and mages and even supports. AD carry is a role that consists of... marksmen... Yea
is the first Mage who could arguably replace an ADC. I've seen a Pantheon and Xin Zhao bot lane destroy a conventional bot lane. You have options, but most people don't want to buck the meta.
and
. just because they can be played with AD items, does not mean it is viable at all (if any) levels of play.
active without moving closer than your auto attack range. in my experience, Thresh "adc" works best with a Blitz support that you are ideally duoing with (communicating in person or through some voice chat).

as well as banner of command and Zz'rots Portal(on PBE).
There are enough tools to push towers + marks men would still deal damage to towers, just not as much.
The fact that they stay the single most powerfull damagesource vs champions would probably keep them relevant even if they couldn't hurt towers at all anyway.